Cellular self-organization by autocatalytic alignment feedback

Tension-driven multi-scale self-organisation in human iPSC-derived muscle fibers

Human muscle is a hierarchically organised tissue with its contractile cells called myofibers packed into large myofiber bundles. Each myofiber contains periodic myofibrils built by hundreds of contractile sarcomeres that generate large mechanical forces. To better understand the mechanisms that coordinate human muscle morphogenesis from tissue to molecular scales, we adopted a simple in vitro system using induced pluripotent stem cell-derived human myogenic precursors. When grown on an unrestricted two-dimensional substrate, developing myofibers spontaneously align and self-organise into higher-order myofiber bundles, which grow and consolidate to stable sizes. Following a transcriptional boost of sarcomeric components, myofibrils assemble into chains of periodic sarcomeres that emerge across the entire myofiber. More efficient myofiber bundling accelerates the speed of sarcomerogenesis suggesting that tension generated by bundling promotes sarcomerogenesis. We tested this hypothesis by directly probing tension and found that tension build-up precedes sarcomere assembly and increases within each assembling myofibril. Furthermore, we found that myofiber ends stably attach to other myofibers using integrin-based attachments and thus myofiber bundling coincides with stable myofiber bundle attachment in vitro. A failure in stable myofiber attachment results in a collapse of the myofibrils. Overall, our results strongly suggest that mechanical tension across sarcomeric components as well as between differentiating myofibers is key to coordinate the multi-scale self-organisation of muscle morphogenesis.

Integer topological defects organize stresses driving tissue morphogenesis

Tissues acquire function and shape via differentiation and morphogenesis. Both processes are driven by coordinating cellular forces and shapes at the tissue scale, but general principles governing this interplay remain to be discovered. Here we report that self-organization of myoblasts around integer topological defects, namely spirals and asters, suffices to establish complex multicellular architectures. In particular, these arrangements can trigger localized cell differentiation or, alternatively, when differentiation is inhibited, they can drive the growth of swirling protrusions. Both localized differentiation and growth of cellular vortices require specific stress patterns. By analysing the experimental velocity and orientational fields through active gel theory, we show that integer topological defects can generate force gradients that concentrate compressive stresses. We reveal these gradients by assessing spatial changes in nuclear volume and deformations of elastic pillars. We propose integer topological defects as mechanical organizing centres controlling differentiation and morphogenesis.

A simple and scalable 3D printing methodology for generating aligned and extended human and murine skeletal muscle tissues

Preclinical biomedical and pharmaceutical research on disease causes, drug targets, and side effects increasingly relies on in vitro models of human tissue. 3D printing offers unique opportunities for generating models of superior physiological accuracy, as well as for automating their fabrication. Towards these goals, we here describe a simple and scalable methodology for generating physiologically relevant models of skeletal muscle. Our approach relies on dual-material micro-extrusion of two types of gelatin hydrogel into patterned soft substrates with locally alternating stiffness. We identify minimally complex patterns capable of guiding the large-scale self-assembly of aligned, extended, and contractile human and murine skeletal myotubes. Interestingly, we find high-resolution patterning is not required, as even patterns with feature sizes of several hundred micrometers is sufficient. Consequently, the procedure is rapid and compatible with any low-cost extrusion-based 3D printer. The generated myotubes easily span several millimeters, and various myotube patterns can be generated in a predictable and reproducible manner. The compliant nature and adjustable thickness of the hydrogel substrates, serves to enable extended culture of contractile myotubes. The method is further readily compatible with standard cell-culturing platforms as well as commercially available electrodes for electrically induced exercise and monitoring of the myotubes.

Explicit calculation method for cell alignment in non-circular geometries

The alignment of spindle-shaped cells in two-dimensional geometries induces singular points called topological defects, at which the alignment angle of the cell cannot be defined. To control defects related to biological roles such as cell apoptosis, calculation methods for predicting the defect positions are required. This study proposes an explicit calculation method for predicting cell alignment and defect positions in non-circular geometries. First, a complex potential is introduced to describe the alignment angles of cells, which is used to derive an explicit formula for cell alignment in a unit disc. Then, the derived formula for the unit disc is extended to the case for non-circular geometries using a numerical conformal mapping. Finally, the complex potential allows a calculation of the Frank elastic energy, which can be minimized with respect to the defect positions to predict their equilibrium state in the geometry. The proposed calculation method is used to demonstrate a numerical prediction of multiple defects in circular and non-circular geometries, which are consistent with previous experimental results.

Multiscale-Engineered Muscle Constructs: PEG Hydrogel Micro-Patterning on an Electrospun PCL Mat Functionalized with Gold Nanoparticles

The development of new, viable, and functional engineered tissue is a complex and challenging task. Skeletal muscle constructs have specific requirements as cells are sensitive to the stiffness, geometry of the materials, and biological micro-environment. The aim of this study was thus to design and characterize a multi-scale scaffold and to evaluate it regarding the differentiation process of C2C12 skeletal myoblasts. The significance of the work lies in the microfabrication of lines of polyethylene glycol, on poly(ε-caprolactone) nanofiber sheets obtained using the electrospinning process, coated or not with gold nanoparticles to act as a potential substrate for electrical stimulation. The differentiation of C2C12 cells was studied over a period of seven days and quantified through both expression of specific genes, and analysis of the myotubes’ alignment and length using confocal microscopy. We demonstrated that our multiscale bio-construct presented tunable mechanical properties and supported the different stages skeletal muscle, as well as improving the parallel orientation of the myotubes with a variation of less than 15°. These scaffolds showed the ability of sustained myogenic differentiation by enhancing the organization of reconstructed skeletal muscle. Moreover, they may be suitable for applications in mechanical and electrical stimulation to mimic the muscle’s physiological functions.

Substrate-dependent control of the chiral orientation of mesenchymal stem cells: image-based quantitative profiling

Stem-cell (SC) chirality or left-right (LR) asymmetry is an essential attribute, observed during tissue regeneration. The ability to control the LR orientation of cells by biophysical manipulation is a promising approach for recapitulating their inherent function. Despite remarkable progress in tissue engineering, the development of LR chirality in SCs has been largely unexplored. Here, we demonstrate the role of substrate stiffness on the LR asymmetry of cultured mesenchymal stem cells (MSCs). We found that MSCs acquired higher asymmetricity when cultured on stiffer PCL/collagen matrices. To confirm cellular asymmetry, different parameters such as the aspect ratio, orientation angle and intensity of polarized proteins (Par) were investigated. The results showed a significant (p < 0.01) difference in the average orientation angle, the cellular aspect ratio, and the expression of actin and Par proteins in MSCs cultured on matrices with different stiffnesses. Furthermore, a Gaussian support-vector machine was applied to classify cells cultured on both (2% and 10% PCL/Collagen) matrices, with a resulting accuracy of 96.2%. To the best of our knowledge, this study is the first that interrelates and quantifies MSC asymmetricity with matrix properties using a simple 2D model.

An in vitro model using spheroids-laden nanofibrous structures for attaining high degree of myoblast alignment and differentiation

A spheroid is an aggregation of single cells with structural and functional characteristics similar to those of 3D native tissues, and it has been utilized as one of the typical in vitro three-dimensional (3D) cell models. Scaffold-free spheroids provide outstanding reflection of tissue complexity in a 3D in vivo-like environment, but they can neither fabricate realistic macroscale 3D complex structures without avoiding necrosis nor receive direct external stimuli (i.e., stimuli from mechanical or topographical cues). Here, we propose a spheroid-laden electrospinning process to obtain in vitro model achieved using the synergistic effect of the unique bioactive components provided by the spheroids and stimulating effects provided by the aligned nanofibers.

Methods: To show the functional activity of the spheroid-laden structures, we used myoblast-spheroids to obtain skeletal muscle, comprising highly aligned myotubes, utilizing an uniaxially arranged topographical cue. The spheroid-electrospinning was used to align spheroids directly by embedding them in aligned alginate nanofibers, which were controlled with various materials and processing parameters.

Results: The spheroids laden in the alginate nanofibers showed high cell viability (>90%) and was compared with that of a cell-laden alginate nanofiber that was electrospun with single cells. Consequently, the spheroids laden in the aligned nanofibers showed a significantly higher degree of myotube formation and maturation.

Conclusion: Results suggested that the in vitro model using electrospun spheroids could potentially be employed to understand myogenic responses for various in vitro drug tests.

Towards an analytical description of active microswimmers in clean and in surfactant-covered drops

Geometric confinements are frequently encountered in the biological world and strongly affect the stability, topology, and transport properties of active suspensions in viscous flow. Based on a far-field analytical model, the low-Reynolds-number locomotion of a self-propelled microswimmer moving inside a clean viscous drop or a drop covered with a homogeneously distributed surfactant, is theoretically examined. The interfacial viscous stresses induced by the surfactant are described by the well-established Boussinesq-Scriven constitutive rheological model. Moreover, the active agent is represented by a force dipole and the resulting fluid-mediated hydrodynamic couplings between the swimmer and the confining drop are investigated. We find that the presence of the surfactant significantly alters the dynamics of the encapsulated swimmer by enhancing its reorientation. Exact solutions for the velocity images for the Stokeslet and dipolar flow singularities inside the drop are introduced and expressed in terms of infinite series of harmonic components. Our results offer useful insights into guiding principles for the control of confined active matter systems and support the objective of utilizing synthetic microswimmers to drive drops for targeted drug delivery applications.

Large-scale spontaneous self-organization and maturation of skeletal muscle tissues on ultra-compliant gelatin hydrogel substrates

Cellular self-organization is the fundamental driving force behind the complex architectures of native tissue. Yet, attempts at replicating native tissue architectures in vitro often involve complex micro-fabrication methods and materials. While impressive progress has been made within engineered models of striated muscle, the wide adaptation of these models is held back by the need for specific tools and knowhow. In this report, we show that C2C12 myoblasts spontaneously organize into highly aligned myotube tissues on the mm to cm scale, when cultured on sufficiently soft yet fully isotropic gelatin hydrogel substrates. Interestingly, we only observed this phenomenon for hydrogels with Young’s modulus of 6 kPa and below. For slightly more rigid compositions, only local micrometer-scale myotube organization was observed, similar to that seen in conventional polystyrene dishes. The hydrogel-supported myotubes could be cultured for multiple weeks and matured into highly contractile phenotypes with notable upregulation of myosin heavy chain, as compared to myotubes developed in conventional petri dishes. The procedure for casting the ultra-soft gelatin hydrogels is straight forward and compatible with standardized laboratory tools. It may thus serve as a simple, yet versatile, approach to generating skeletal muscle tissue of improved physiological relevance for applied and basic research.

Engineering macroscale cell alignment through coordinated toolpath design using support-assisted 3D bioprinting

Aligned cells provide direction-dependent mechanical properties that influence biological and mechanical function in native tissues. Alignment techniques such as casting and uniaxial stretching cannot fully replicate the complex fibre orientation of native tissue such as the heart. In this study, bioprinting is used to direct the orientation of cell alignment. A 0°-90° grid structure was printed to assess the robustness of the support-assisted bioprinting technique. The variation in the angles of the grid pattern is designed to mimic the differences in fibril orientation of native tissues, where angles of cell alignment vary across the different layers. Through bioprinting of a cell-hydrogel mixture, C2C12 cells displayed directed alignment along the longitudinal axis of printed struts. Cell alignment is induced through firstly establishing structurally stable constructs (i.e. distinct 0°-90° structures) and secondly, allowing cells to dynamically remodel the bioprinted construct. Herein reports a method of inducing a macroscale level of controlled cell alignment with angle variation. This was not achievable both in terms of methods (i.e. conventional alignment techniques such as stretching and electrical stimulation) and magnitude (i.e. hydrogel features with less than 100 µm features).

Building Complex Life Through Self-Organization

Cells are inherently conferred with the ability to self-organize into the tissues and organs comprising the human body. Self-organization can be recapitulated in vitro and recent advances in the organoid field are just one example of how we can generate small functioning elements of organs. Tissue engineers can benefit from the power of self-organization and should consider how they can harness and enhance the process with their constructs. For example, aggregates of stem cells and tissue-specific cells benefit from the input of carefully selected biomolecules to guide their differentiation toward a mature phenotype. This can be further enhanced by the use of technologies to provide a physiological microenvironment for self-organization, enhance the size of the constructs, and enable the long-term culture of self-organized structures. Of importance, conducting self-organization should be limited to fine-tuning and should avoid over-engineering that could counteract the power of inherent cellular self-organization. Impact Statement Self-organization is a powerful innate feature of cells that can be fine-tuned but not over-engineered to create new tissues and organs.

Overlooked? Underestimated? Effects of Substrate Curvature on Cell Behavior

In biological systems, form and function are inherently correlated. Despite this strong interdependence, the biological effect of curvature has been largely overlooked or underestimated, and consequently it has rarely been considered in the design of new cell-material interfaces. This review summarizes current understanding of the interplay between the curvature of a cell substrate and the related morphological and functional cellular response. In this context, we also discuss what is currently known about how, in the process of such a response, cells recognize curvature and accordingly reshape their membrane. Beyond this, we highlight state-of-the-art microtechnologies for engineering curved biomaterials at cell-scale, and describe aspects that impair or improve readouts of the pure effect of curvature on cells.

Nanotopography-responsive myotube alignment and orientation as a sensitive phenotypic biomarker for Duchenne Muscular Dystrophy

Duchenne Muscular Dystrophy (DMD) is a fatal genetic disorder currently having no cure. Here we report that culture substrates patterned with nanogrooves and functionalized with Matrigel (or laminin) present an engineered cell microenvironment to allow myotubes derived from non-diseased, less-affected DMD, and severely-affected DMD human induced pluripotent stem cells (hiPSCs) to exhibit prominent differences in alignment and orientation, providing a sensitive phenotypic biomarker to potentially facilitate DMD drug development and early diagnosis. We discovered that myotubes differentiated from myogenic progenitors derived from non-diseased hiPSCs align nearly perpendicular to nanogrooves, a phenomenon not reported previously. We further found that myotubes derived from hiPSCs of a dystrophin-null DMD patient orient randomly, and those from hiPSCs of a patient carrying partially functional dystrophin align approximately 14° off the alignment direction of non-diseased myotubes. Substrates engineered with micron-scale grooves and/or cell adhesion molecules only interacting with integrins all guide parallel myotube alignment to grooves and lose the ability to distinguish different cell types. Disruption of the interaction between the Dystrophin-Associated-Protein-Complex (DAPC) and laminin by heparin or anti-α-dystroglycan antibody IIH6 disenables myotubes to align perpendicular to nanogrooves, suggesting that this phenotype is controlled by the DAPC-mediated cytoskeleton-extracellular matrix linkage.

Outline-etching image segmentation reveals enhanced cell chirality through intercellular alignment

Cells cultured on micropatterns exhibit a chiral orientation, which may underlie the development of left–right asymmetry in tissue microarchitectures. To investigate this phenomenon, fluorescence staining of nuclei has been used to reveal such orientation. However, for images with high cell density, analysis is difficult because of the overlapping nuclei. Here, we report an image processing method that can acquire cell orientations within dense cell populations. After initial separation based on Boolean addition of binarized images using global and adaptive thresholds, the overlapping nucleus contours in the binarized images were segmented by iteratively etching the outlines of nuclei, which allowed the orientations of each cell to be extracted from densely packed cell clusters. In applying this technique to cultured C2C12 myoblasts in micropatterned stripes on different substrates, we found an enhanced chiral orientation on glass substrate. More important, this enhanced chirality was consistently observed with increased intercellular alignment and independent of cell–cell distance or cell density, suggesting that intercellular alignment plays a role in determining the chiral orientation. By segmenting single cells with intact orientation, this technique offers an automated method for quantitative analysis with improved accuracy, providing an essential tool for studying left–right asymmetry and other morphogenic dynamics in tissue formation.

Regulation of skeletal myotube formation and alignment by nanotopographically controlled cell-secreted extracellular matrix

Skeletal muscle has a well-organized tissue structure comprised of aligned myofibers and an encasing extracellular matrix (ECM) sheath or lamina, within which reside satellite cells. We hypothesize that the organization of skeletal muscle tissues in culture can affect both the structure of the deposited ECM and the differentiation potential of developing myotubes. Furthermore, we posit that cellular and ECM cues can be a strong determinant of myoblast fusion and morphology in 3D tissue culture environments. To test these, we utilized a thermoresponsive nanofabricated substratum to engineer anisotropic sheets of myoblasts which could then be transferred and stacked into multilayered tissues. Within such engineered tissues, we found that myoblasts rapidly sense topography and deposit structurally organized ECM proteins. Furthermore, the initial tissue structure was found to exert significant control over myoblast fusion and eventual myotube organization. These results highlight the importance of ECM structure on myoblast fusion and organization, and provide insights into substrate-mediated control of myotube formation in the development of novel, more effective, engineered skeletal muscle tissues. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 1543-1551, 2018.

MyoScreen, a High-Throughput Phenotypic Screening Platform Enabling Muscle Drug Discovery

Despite the need for more effective drug treatments to address muscle atrophy and disease, physiologically accurate in vitro screening models and higher information content preclinical assays that aid in the discovery and development of novel therapies are lacking. To this end, MyoScreen was developed: a robust and versatile high-throughput high-content screening (HT/HCS) platform that integrates a physiologically and pharmacologically relevant micropatterned human primary skeletal muscle model with a panel of pertinent phenotypic and functional assays. MyoScreen myotubes form aligned, striated myofibers, and they show nerve-independent accumulation of acetylcholine receptors (AChRs), excitation-contraction coupling (ECC) properties characteristic of adult skeletal muscle and contraction in response to chemical stimulation. Reproducibility and sensitivity of the fully automated MyoScreen platform are highlighted in assays that quantitatively measure myogenesis, hypertrophy and atrophy, AChR clusterization, and intracellular calcium release dynamics, as well as integrating contractility data. A primary screen of 2560 compounds to identify stimulators of myofiber regeneration and repair, followed by further biological characterization of two hits, validates MyoScreen for the discovery and testing of novel therapeutics. MyoScreen is an improvement of current in vitro muscle models, enabling a more predictive screening strategy for preclinical selection of the most efficacious new chemical entities earlier in the discovery pipeline process.

Tensile forces drive a reversible fibroblast-to-myofibroblast transition during tissue growth in engineered clefts

Myofibroblasts orchestrate wound healing processes, and if they remain activated, they drive disease progression such as fibrosis and cancer. Besides growth factor signaling, the local extracellular matrix (ECM) and its mechanical properties are central regulators of these processes. It remains unknown whether transforming growth factor-β (TGF-β) and tensile forces work synergistically in up-regulating the transition of fibroblasts into myofibroblasts and whether myofibroblasts undergo apoptosis or become deactivated by other means once tissue homeostasis is reached. We used three-dimensional microtissues grown in vitro from fibroblasts in macroscopically engineered clefts for several weeks and found that fibroblasts transitioned into myofibroblasts at the highly tensed growth front as the microtissue progressively closed the cleft, in analogy to closing a wound site. Proliferation was up-regulated at the growth front, and new highly stretched fibronectin fibers were deposited, as revealed by fibronectin fluorescence resonance energy transfer probes. As the tissue was growing, the ECM underneath matured into a collagen-rich tissue containing mostly fibroblasts instead of myofibroblasts, and the fibronectin fibers were under reduced tension. This correlated with a progressive rounding of cells from the growth front inward, with decreased α-smooth muscle actin expression, YAP nuclear translocation, and cell proliferation. Together, this suggests that the myofibroblast phenotype is stabilized at the growth front by tensile forces, even in the absence of endogenously supplemented TGF-β, and reverts into a quiescent fibroblast phenotype already 10 μm behind the growth front, thus giving rise to a myofibroblast-to-fibroblast transition. This is the hallmark of reaching prohealing homeostasis.

Hybrid Randomly Electrospun Poly(lactic-co-glycolic acid):Poly(ethylene oxide) (PLGA:PEO) Fibrous Scaffolds Enhancing Myoblast Differentiation and Alignment

Cellular responses are regulated by their microenvironments, and engineered synthetic scaffolds can offer control over different microenvironment properties. This important relationship can be used as a tool to manipulate cell fate and cell responses for different biomedical applications. We show for the first time in this study how blending of poly(ethylene oxide) (PEO) to poly(lactic-co-glycolic acid) (PLGA) fibers to yield hybrid scaffolds changes the physical and mechanical properties of PLGA fibrous scaffolds and in turn affects cellular response. For this purpose we employed electrospinning to create fibrous scaffolds mimicking the basic structural properties of the native extracellular matrix. We introduced PEO to PLGA electrospun fibers by spinning a blend of PLGA:PEO polymer solutions in different ratios. PEO served as a sacrificial component within the fibers upon hydration, leading to pore formation in the fibers, fiber twisting, increased scaffold disintegration, and hydrophilicity, decreased Young's modulus, and significantly improved strain at break of initially electrospun scaffolds. We observed that the blended PLGA:PEO fibrous scaffolds supported myoblast adhesion and proliferation and resulted in increased myotube formation and self-alignment, when compared to PLGA-only scaffolds, even though the scaffolds were randomly oriented. The 50:50 PLGA:PEO blended scaffold showed the most promising results in terms of mechanical properties, myotube formation, and alignment, suggesting an optimal microenvironment for myoblast differentiation from the PLGA:PEO blends tested. The explored approach for tuning fiber properties can easily extend to other polymeric scaffolds and provides a valuable tool to engineer fibrillar microenvironments for several biomedical applications.

Image analysis of self-organized multicellular patterns

Analysis of multicellular patterns is required to understand tissue organizational processes. By using a multi-scale object oriented image processing method, the spatial information of cells can be extracted automatically. Instead of manual segmentation or indirect measurements, such as general distribution of contrast or flow, the orientation and distribution of individual cells is extracted for quantitative analysis. Relevant objects are identified by feature queries and no low-level knowledge of image processing is required.

Substrate Stiffness Regulates the Development of Left-Right Asymmetry in Cell Orientation

Left-right (LR) asymmetry of tissue/organ structure is a morphological feature essential for many tissue functions. The ability to incorporate the LR formation in constructing tissue/organ replacement is important for recapturing the inherent tissue structure and functions. However, how LR asymmetry is formed remains largely underdetermined, which creates significant hurdles to reproduce and regulate the formation of LR asymmetry in an engineering context. Here, we report substrate rigidity functioning as an effective switch that turns on the development of LR asymmetry. Using micropatterned cell-adherent stripes on rigid substrates, we found that cells collectively oriented at a LR-biased angle relative to the stripe boundary. This LR asymmetry was initiated by a LR-biased migration of cells at stripe boundary, which later generated a velocity gradient propagating from stripe boundary to the center. After a series of cell translocations and rotations, ultimately, an LR-biased cell orientation within the micropatterned stripe was formed. Importantly, this initiation and propagation of LR asymmetry was observed only on rigid but not on soft substrates, suggesting that the LR asymmetry was regulated by rigid substrate probably through the organization of actin cytoskeleton. Together, we demonstrated substrate rigidity as a determinant factor that mediates the self-organizing LR asymmetry being unfolded from single cells to multicellular organization. More broadly, we anticipate that our findings would pave the way for rebuilding artificial tissue constructs with inherent LR asymmetry in the future.

Cellular Architecture Regulates Collective Calcium Signaling and Cell Contractility

A key feature of multicellular systems is the ability of cells to function collectively in response to external stimuli. However, the mechanisms of intercellular cell signaling and their functional implications in diverse vascular structures are poorly understood. Using a combination of computational modeling and plasma lithography micropatterning, we investigate the roles of structural arrangement of endothelial cells in collective calcium signaling and cell contractility. Under histamine stimulation, endothelial cells in self-assembled and microengineered networks, but not individual cells and monolayers, exhibit calcium oscillations. Micropatterning, pharmacological inhibition, and computational modeling reveal that the calcium oscillation depends on the number of neighboring cells coupled via gap junctional intercellular communication, providing a mechanistic basis of the architecture-dependent calcium signaling. Furthermore, the calcium oscillation attenuates the histamine-induced cytoskeletal reorganization and cell contraction, resulting in differential cell responses in an architecture-dependent manner. Taken together, our results suggest that endothelial cells can sense and respond to chemical stimuli according to the vascular architecture via collective calcium signaling.

Probing Leader Cells in Endothelial Collective Migration by Plasma Lithography Geometric Confinement

When blood vessels are injured, leader cells emerge in the endothelium to heal the wound and restore the vasculature integrity. The characteristics of leader cells during endothelial collective migration under diverse physiological conditions, however, are poorly understood. Here we investigate the regulation and function of endothelial leader cells by plasma lithography geometric confinement generated. Endothelial leader cells display an aggressive phenotype, connect to follower cells via peripheral actin cables and discontinuous adherens junctions, and lead migrating clusters near the leading edge. Time-lapse microscopy, immunostaining, and particle image velocimetry reveal that the density of leader cells and the speed of migrating clusters are tightly regulated in a wide range of geometric patterns. By challenging the cells with converging, diverging and competing patterns, we show that the density of leader cells correlates with the size and coherence of the migrating clusters. Collectively, our data provide evidence that leader cells control endothelial collective migration by regualting the migrating clusters.

Direct cellular organization with ring-shaped composite polymers and glass substrates for urethral sphincter tissue engineering

We introduce the substrates of composite materials for sphincter tissue engineering and demonstrate the mechanisms of how dimensions, curvature and parallelism of constraints affect cellular organization.

Selectively manipulable acoustic-powered microswimmers

Selective actuation of a single microswimmer from within a diverse group would be a first step toward collaborative guided action by a group of swimmers. Here we describe a new class of microswimmer that accomplishes this goal. Our swimmer design overcomes the commonly-held design paradigm that microswimmers must use non-reciprocal motion to achieve propulsion; instead, the swimmer is propelled by oscillatory motion of an air bubble trapped within the swimmer's polymer body. This oscillatory motion is driven by the application of a low-power acoustic field, which is biocompatible with biological samples and with the ambient liquid. This acoustically-powered microswimmer accomplishes controllable and rapid translational and rotational motion, even in highly viscous liquids (with viscosity 6,000 times higher than that of water). And by using a group of swimmers each with a unique bubble size (and resulting unique resonance frequencies), selective actuation of a single swimmer from among the group can be readily achieved.

Probing mechanoregulation of neuronal differentiation by plasma lithography patterned elastomeric substrates

Cells sense and interpret mechanical cues, including cell-cell and cell-substrate interactions, in the microenvironment to collectively regulate various physiological functions. Understanding the influences of these mechanical factors on cell behavior is critical for fundamental cell biology and for the development of novel strategies in regenerative medicine. Here, we demonstrate plasma lithography patterning on elastomeric substrates for elucidating the influences of mechanical cues on neuronal differentiation and neuritogenesis. The neuroblastoma cells form neuronal spheres on plasma-treated regions, which geometrically confine the cells over two weeks. The elastic modulus of the elastomer is controlled simultaneously by the crosslinker concentration. The cell-substrate mechanical interactions are also investigated by controlling the size of neuronal spheres with different cell seeding densities. These physical cues are shown to modulate with the formation of focal adhesions, neurite outgrowth, and the morphology of neuroblastoma. By systematic adjustment of these cues, along with computational biomechanical analysis, we demonstrate the interrelated mechanoregulatory effects of substrate elasticity and cell size. Taken together, our results reveal that the neuronal differentiation and neuritogenesis of neuroblastoma cells are collectively regulated via the cell-substrate mechanical interactions.

Extracellular-controlled breast cancer cell formation and growth using non-UV patterned hydrogels via optically-induced electrokinetics

The culturing of cancer cells on micropatterned substrates can provide insight into the factors of the extracellular environment that enable the control of cell growth. We report here a novel non-UV-based technique to quickly micropattern a poly-(ethylene) glycol diacrylate (PEGDA)-based hydrogel on top of modified glass substrates, which were then used to control the growth patterns of breast cancer cells. Previously, the fabrication of micropatterned substrates required relatively complicated steps, which made it impractical for researchers to rapidly and systematically investigate the effects of different cell growth patterns. The technique presented herein operates on the principle of optically-induced electrokinetics (OEKs) and uses computer-generated projection light patterns to dynamically pattern the hydrogel on a hydrogenated amorphous silicon (a-Si:H) thin-film, atop an indium tin oxide (ITO) glass substrate. This technique allows us to pattern lines, circles, pentagons, and more complex shapes in the hydrogel with line widths below 3 μm and thicknesses of up to 6 μm within 8 s by simply controlling the projected illumination pattern and applying an appropriate AC voltage between the two ITO glass substrates. After separating the glass substrates to expose the patterned hydrogel, we experimentally demonstrate that MCF-7 breast cancer cells will adhere to the bare a-Si:H surface, but not to the hydrogel patterned in various geometric shapes and sizes. Theoretical analysis and finite-element model simulations reveal that the dominant OEK forces in our technique are the dielectrophoresis (DEP) force and the electro-osmosis force, which enhance the photo-initiated cross-linking reaction in the hydrogel. Our preliminary cultures of breast cancer cells demonstrate that this reported technique could be applied to effectively confine the growth of cancer cells on a-Si:H surfaces and affect individual cell geometry during their growth.

Mechanically induced intercellular calcium communication in confined endothelial structures

Calcium signaling in the diverse vascular structures is regulated by a wide range of mechanical and biochemical factors to maintain essential physiological functions of the vasculature. To properly transmit information, the intercellular calcium communication mechanism must be robust against various conditions in the cellular microenvironment. Using plasma lithography geometric confinement, we investigate mechanically induced calcium wave propagation in networks of human umbilical vein endothelial cells organized. Endothelial cell networks with confined architectures were stimulated at the single cell level, including using capacitive force probes. Calcium wave propagation in the network was observed using fluorescence calcium imaging. We show that mechanically induced calcium signaling in the endothelial networks is dynamically regulated against a wide range of probing forces and repeated stimulations. The calcium wave is able to propagate consistently in various dimensions from monolayers to individual cell chains, and in different topologies from linear patterns to cell junctions. Our results reveal that calcium signaling provides a robust mechanism for cell-cell communication in networks of endothelial cells despite the diversity of the microenvironmental inputs and complexity of vascular structures.

Calcium wave propagation in networks of endothelial cells: model-based theoretical and experimental study

In this paper, we present a combined theoretical and experimental study of the propagation of calcium signals in multicellular structures composed of human endothelial cells. We consider multicellular structures composed of a single chain of cells as well as a chain of cells with a side branch, namely a “T” structure. In the experiments, we investigate the result of applying mechano-stimulation to induce signaling in the form of calcium waves along the chain and the effect of single and dual stimulation of the multicellular structure. The experimental results provide evidence of an effect of architecture on the propagation of calcium waves. Simulations based on a model of calcium-induced calcium release and cell-to-cell diffusion through gap junctions shows that the propagation of calcium waves is dependent upon the competition between intracellular calcium regulation and architecture-dependent intercellular diffusion.

Calcium wave signal has been found in a wide variety of cell types. Over the last years, a large number of calcium experiments have shown that calcium signal is not only an intracellular regulator but is also able to be transmitted to surrounding cells as intercellular signal. This paper focuses on the development of an approach with complementary integration of theoretical and experimental methods for studying the multi-level interactions in multicellular architectures and their effect on collective cell dynamic behavior. We describe new types of higher-order (across structure) behaviors arising from lower-order (within cells) phenomena, and make predictions concerning the mechanisms underlying the dynamics of multicellular biological systems. The theoretical approach describes numerically the dynamics of non-linear behavior of calcium-based signaling in model networks of cells. Microengineered, geometrically constrained networks of human umbilical vein endothelial cells (HUVEC) serve as platforms to arbitrate the theoretical predictions in terms of the effect of network topology on the spatiotemporal characteristics of emerging calcium signals.

Preferred mitotic orientation in pattern formation by vascular mesenchymal cells

Cellular self-organization is essential to physiological tissue and organ development. We previously observed that vascular mesenchymal cells, a multipotent subpopulation of aortic smooth muscle cells, self-organize into macroscopic, periodic patterns in culture. The patterns are produced by cells gathering into raised aggregates in the shape of nodules or ridges. To determine whether these patterns are accounted for by an oriented pattern of cell divisions or postmitotic relocation of cells, we acquired time-lapse, videomicrographic phase-contrast, and fluorescence images during self-organization. Cell division events were analyzed for orientation of daughter cells in mitoses during separation and their angle relative to local cell alignment, and frequency distribution of the mitotic angles was analyzed by both histographic and bin-free statistical methods. Results showed a statistically significant preferential orientation of daughter cells along the axis of local cell alignment as early as day 8, just before aggregate formation. This alignment of mitotic axes was also statistically significant at the time of aggregate development (day 11) and after aggregate formation was complete (day 15). Treatment with the nonmuscle myosin II inhibitor, blebbistatin, attenuated alignment of mitotic orientation, whereas Rho kinase inhibition eliminated local cell alignment, suggesting a role for stress fiber orientation in this self-organization. Inhibition of cell division using mitomycin C reduced the macroscopic pattern formation. Time-lapse monitoring of individual cells expressing green fluorescent protein showed postmitotic movement of cells into neighboring aggregates. These findings suggest that polarization of mitoses and postmitotic migration of cells both contribute to self-organization into periodic, macroscopic patterns in vascular stem cells.

Directing tissue morphogenesis via self-assembly of vascular mesenchymal cells

Rebuilding injured tissue for regenerative medicine requires technologies to reproduce tissue/biomaterials mimicking the natural morphology. To reconstitute the tissue pattern, current approaches include using scaffolds with specific structure to plate cells, guiding cell spreading, or directly moving cells to desired locations. However, the structural complexity is limited. Also, the artificially-defined patterns are usually disorganized by cellular self-organization in the subsequent tissue development, such as cell migration and cell-cell communication. Here, by working in concert with cellular self-organization rather than against it, we experimentally and mathematically demonstrate a method which directs self-organizing vascular mesenchymal cells (VMCs) to assemble into desired multicellular patterns. Incorporating the inherent chirality of VMCs revealed by interfacing with microengineered substrates and VMCs' spontaneous aggregation, differences in distribution of initial cell plating can be amplified into the formation of striking radial structures or concentric rings, mimicking the cross-sectional structure of liver lobules or osteons, respectively. Furthermore, when co-cultured with VMCs, non-pattern-forming endothelial cells (ECs) tracked along the VMCs and formed a coherent radial or ring pattern in a coordinated manner, indicating that this method is applicable to heterotypical cell organization.